This work aimed at determining empirical correlations between the final frozen product ice crystals' characteristics (mean size and shape) and ultrasound assisted freezing operating conditions (supercooling and acoustic power). For this purpose 10% w/w mannitol aqueous solution samples contained in vials were frozen by means of a refrigerated shelf coupled with an ultrasonic device able to induce ice nucleation. Ice crystals' were observed using reflected light optical microscopy over samples' transversal and longitudinal sections in a cold chamber. A response surface analysis based on a five levels composite experimental design had been carried out. It was found that increasing supercooling and acoustic power resulted in decreasing ice crystals' mean size and increasing their mean circularity. (C) 2010 Elsevier Ltd. All rights reserved.